Nowadays, a liquid crystal display has been used commonly in personal computers, PDAs (personal digital assistants), mobile phones, and the like. As such devices become widespread, the chances of displaying information on a liquid crystal display screen in public are increasing.
The following is a brief description of a configuration of a conventional liquid crystal display. The liquid crystal display has a liquid crystal layer sandwiched between a pair of translucent substrates. On one of the translucent substrates (i.e., an active matrix substrate), data signal lines to which display data is applied and gate signal lines for driving active elements described below are arranged in matrix. At each intersection of the data signal lines and the gate signal lines, the active element such as a thin film transistor (TFT) to which a picture element electrode is connected is connected. The picture element electrodes are arranged on the translucent substrate in matrix. On the other translucent substrate (i.e., a counter substrate), a color filter layer including color filters of red, green, and blue and a black matrix for preventing light leakage between these color filters is formed. On the color filter layer, a transparent conductive film functioning as a counter electrode for driving liquid crystal is laminated.
Further, on each of the active matrix substrate and the counter substrate, a liquid crystal alignment film made of polyimide or the like is formed under predetermined conditions, followed by an alignment treatment such as rubbing in accordance with a desired viewing angle. Then, a spacer such as plastic beads for forming a cell gap is arranged on one of the substrates, and an adhesive such as a thermosetting sealant is applied to the periphery of a display area on the other substrate. Thereafter, the active matrix substrate and the counter substrate are bonded together with the adhesive, and a liquid crystal material is injected between the substrates by a vacuum injection method or the like, followed by sealing of an injection port. In this manner, a liquid crystal display element is obtained. Meanwhile, a liquid crystal display is required to have display qualities such as high brightness, high contrast, high definition, and a wide viewing angle, which are being improved rapidly. In particular, a conventional TN (twisted nematic) mode, STS (super twisted nematic) mode, etc. are disadvantageous in that they lead to a narrow viewing angle. In order to overcome this disadvantage, liquid crystal modes capable of providing a wide viewing angle such as MVA (multi-domain vertical alignment), CPA (continuous pinwheel alignment), IPS (in-plane switching), FFS (fringe field switching), and OCB (optically compensated birefringence) are being developed. However, the development of such liquid crystal modes capable of providing a wide viewing angle has led to another problem.
That is, when a user observes a screen of a mobile personal computer, a PDA, a mobile phone, or the like having a wide viewing angle in public, the wide viewing angle allows others to look at the screen from a viewing angle other than the front direction for the user, e.g., from a lateral direction. This may result in a leakage of private information. Nowadays, combined with enforcement of the Personal Information Protection Law, there has been an increasing demand from users for privacy protection by making a viewing angle narrower contrary to a conventional tendency. To this end, it has been popular to attach a viewing angle adjusting film for making a viewing angle narrower to a screen, which, however, always makes the viewing angle narrower. Thus, it also has been demanded to switch dynamically between a wide viewing angle inherent in a display and a narrow viewing angle in favor of privacy protection according to need. In view of this, a display in which a viewing angle control device capable of switching dynamically between a wide viewing angle and a narrow viewing angle by using birefringence of liquid crystal is provided on a display device for displaying an image has been proposed.
Various systems of viewing angle control devices using liquid crystal have been proposed, such as a light dispersion system using polymer-dispersed liquid crystal (Japanese Patent No. 3481741 and JP 7(1995)-333640 A), a light absorption system using guest host liquid crystal (JP 10(1998)-197844 A), a TN mode (JP 10(1998)-268251 A) an IPS mode (JP 11(1999)-30783 A), a hybrid alignment mode (JP 2005-275342 A), and a homogeneous alignment mode OP 2005-316407 A).
In particular, a viewing angle control device using homogeneously aligned liquid crystal as disclosed in JP 2005-316407 A has many advantages such as a simple manufacturing process, capability of switching between a wide viewing angle and a narrow viewing angle, and relatively high transmittance in a normal direction. With reference to FIGS. 12A and 12B, a configuration of the viewing angle control device using the homogeneously aligned liquid crystal will be described. FIG. 12A is a schematic view showing a state of liquid crystal molecules during a wide viewing angle mode of the viewing angle control device, and FIG. 12B is a schematic view showing a state of the liquid crystal molecules during a narrow viewing angle mode of the viewing angle control device. In FIGS. 12A and 12B, reference numerals 91 and 92 denote a pair of translucent substrates, and reference numeral 93 denotes the liquid crystal molecules of a liquid crystal layer sandwiched between the translucent substrates 91 and 92. Arrows denoted by reference numerals 94 and 95 represent the polarization-transmission axes of polarizers laminated on the translucent substrates 91 and 92. The polarization-transmission axes 94 and 95 are parallel to each other. Alignment films on the translucent substrates 91 and 92 are subjected to a so-called antiparallel rubbing process in which the rubbing directions are substantially parallel to the polarization-transmission axes 94 and 95 and opposite to each other between the translucent substrates 91 and 92.
In this viewing angle control device, when no voltage is applied to the liquid crystal layer, the liquid crystal molecules 93 are oriented with their long molecular axes aligning substantially parallel to the polarization-transmission axes 94 and 95, as shown in FIG. 12A. Thus, even if a view point is inclined laterally with respect to the panel from the normal direction to the substrates (i.e., in a plane perpendicular to the polarization-transmission axes 94 and 95), no phase difference occurs in the liquid crystal layer, so that a display device on which the viewing angle control device is laminated can maintain a wide viewing angle. This state is referred to as a wide viewing angle state.
On the other hand, when a predetermined voltage is applied to the liquid crystal layer, the liquid crystal molecules 93 rise at an inclination angle θ with respect to the substrate surface in accordance with the applied voltage, as shown in FIG. 12B. Consequently, if a view point is inclined laterally with respect to the panel from the normal direction to the substrates, a phase difference occurs in the liquid crystal layer. When the inclination of the view point reaches a certain angle, a black display is provided depending on the VT (voltage-transmittance) characteristics, as shown in FIG. 13. Thus, a display of the display device is shielded by the black display of the viewing angle control device in the lateral direction other than the normal direction to the substrates (i.e., the front). This state is referred to as a narrow viewing angle state.